For illustrating background art in the field of pulse laser technique, reference is made to the following prior art documents.    [1] D. Bauer et al. in “Opt. Express”, vol. 20, 2012, p. 9698;    [2] C. Saraceno et al. in “Opt. Express”, vol. 20, 2012, p. 23535;    [3] O. Pronin et al. in “Opt. Lett.”, vol. 36, 2011, p. 4746;    [4] S. A. Meyer et al. in “Eur. Phys. J. D”, vol. 48, 2008, p. 1926;    [5] T. Balcunias et al. in “Opt. Lett.”, vol. 36, 2011, p. 3242    [6] H. R. Telle et al. in “Appl. Phys. B”, vol. 69, 1999, p. 327;    [7] U.S. Pat. No. 7,474,457;    [8] D. Noordegraaf, “Fused combiners for photonic crystal fibers” Ph.D. Thesis 2012;    [9] EP 1 081 819;    [10] U.S. Pat. No. 6,785,303 B1; and    [11] U.S. Pat. No. 6,850,543 B2.
Various applications of pulse laser devices (pulse laser oscillators), e. g. in non-linear spectroscopy and high field applications require a stabilization of the laser output. Intensity fluctuations may be caused by a noisy oscillator, e. g. due to an intrinsic mode-locking behavior or cooling of the laser medium. Furthermore, in the fields of femtosecond and sub-femtosecond physics and frequency comb generation, wherein laser pulses are used in measurements with the demand for maximal temporal and/or spectral resolution, a carrier envelope phase (CEP) control is required for minimizing effects of CEP noise. Depending on the application of the pulse laser device, each of the intensity stabilisation, in particular intensity noise reduction, and CEP stabilisation can be implemented alone, or both can be provided in parallel.
It is generally known that thin disk (TD) laser technique allows creating sub-ps pulses with high average powers and high energies [1, 2]. A pulse laser oscillator is described e. g. in [3], wherein 45 W, 1.1 μJ and 250 fs output pulses are created with a pump power of about 220 W. Low power solid state oscillators and fiber oscillators can be CEP stabilized by controlling a supply current of a pump diode[4, 5].
According to [4], a bulk laser crystal of a pulse laser oscillator is pumped with two Bragg grating stabilized pump laser diodes. The pump laser diodes are arranged with a standard geometry wherein the bulk crystal is end-pumped from both sides for increasing the pump power. One of the pump laser diodes is stabilized by modulating the supply current thereof in dependency on CEP of the output pulses. However, the stabilization effect obtained by Meyer et al. is limited due to a very noisy operation of the oscillator, which results from a mutual interaction of the laser diodes pointed towards each other. Moreover, this geometry is not applicable to high-power thin-disk lasers due to the low single-pass-absorption in a disk medium.
Applying the concept of [4] to high power solid state oscillators would be highly desirable. However, this requires fulfilling the following crucial criteria for a constant current power supply for CEP stabilization:
a) high current stability;
b) analogue control of drive current; and
c) broad bandwidth (more than several tens of kHz) of the analogue control.
Practical test of available high power constant current power supplies (>400 W) have shown that they do not satisfy all three criteria a) to c). It is very difficult to steer high currents on the one hand with a high stability and on the other hand with high dynamics. It has been found that this problem cannot be solved by combining low power current supplies connected in parallel or in series. It is difficult to avoid crosstalk between the devices, and this approach would by expensive and bulky.
Optically combining the output of many laser diodes is inherent to the operation of diode bars and diode stacks, resp. It has been proposed to combine the output of the diodes into a single fiber, e. g. using a monolithic all-fiber combiner [8]. The conventional fiber combiners have limitations in terms of the capability of combining beams with different properties, e. g. different power. Furthermore, a free space combination of the diode output has been described [9].